Filler suspension and its use in the manufacture of paper

ABSTRACT

Provided herein are filler suspensions comprising swollen cationic starch, anionic, water-soluble polymer and filler particles. Pulp furnishes comprising the filler suspensions and paper comprising the pulp furnish are also provided. Processes for making the filler suspensions and processes for their use in manufacturing paper are also provided.

RELATED APPLICATIONS

This application is a continuation of PCT application Serial Number PCT/2012/059117, filed on 5 Oct. 2012, the PCT application being incorporated by reference as if fully set forth herein.

FIELD

Provided herein is a filler suspension comprising swollen cationic starch and an anionic, water-soluble polymer, methods of its use for the manufacture of paper, and paper and paper products comprising the suspension.

BACKGROUND

In the manufacture of filled papers, filler slurry is conventionally added to a pulp suspension before it is transferred to the forming section of a paper machine. A retention aid or retention aid system comprising several components is generally added to the pulp/filler suspension (also known as the furnish) to retain the filler in the resulting paper sheet.

Adding filler to paper can provide numerous improvements in sheet properties, including improved opacity, brightness, feel, and print definition. Further, when the filler is cheaper than the pulp, addition of filler to the sheet results in cost savings due to the replacement of pulp fiber by filler. These savings can be substantial when low cost fillers, such as precipitated calcium carbonate (PCC), are used to replace expensive chemical pulp fibers. Moreover, filled paper can be easier to dry than paper with no filler and, as a result, a paper machine can run faster with less steam consumption, which can further reduce costs and improve productivity.

For a given sheet weight, however, there are limits to the amount of filler that can be added. The strength of the resulting paper is a prime factor, together with paper machine efficiency, limiting filler content, although other factors such as retention, drainage and sizing are also a consideration.

Making paper with a high filler content requires an efficient retention aid system. The retention aid should provide good filler retention under the high shear and turbulence occurring in the paper manufacturing process and should improve drainage without impairing formation. The retention aid chemicals are generally added to the furnish prior to or at the inlet to the headbox of the paper machine. The retention aids are one, two or three component chemical additives that improve filler and fines retention by a bridging and/or flocculation mechanism. The chemicals help attach the filler particles and fines (small fibrous fragments) to the long fibers or cause their aggregation into larger flocculated particles which are more easily retained in the web. In order to create the attachment and flocculation, the chemicals must adsorb on the surfaces of the fillers, fines and fibers. The degree of adsorption of chemicals and the attachment forces are influenced by many things including furnish cleanliness and furnish chemistry, the properties of the added chemicals, the level of shear in the papermaking process and the contact time between the retention aids and the furnish components.

Paper strength is inevitably reduced by replacement of fiber with filler, not only because there are fewer fibers in the sheet, which reduces the number of fiber-fiber bonds, but also because the presence of the filler reduces contact between the remaining fibers. Filler particles do not bond among themselves and their location at the fiber-fiber bonded area prevents hydrogen bonding from occurring between the pulp fibers. As a result, retaining high amounts of filler produces a weaker sheet that can break more easily on the paper machine, size press, coater, winders and printing presses. Weaker fiber-fiber bonding also decreases the surface strength of the paper, causing a reduction in pick resistance and an increase in linting. Poor bonding of filler particles in the fibrous structure can also increase dusting in the pressroom.

In general, all inorganic fillers generally used in paper making such as, without limitation, clay, ground calcium carbonate (GCC), PCC, chalk, talc, titanium dioxide and precipitated calcium sulphate (PCS) are known to impair paper strength and increase demand for chemicals. Fillers with high surface areas, such as small scalenohedral PCC, have substantial negative effects on strength and increase the chemical demand for additives used for strength, sizing and retention. Due to its shape, narrow particle size distribution, and high surface area, PCC has a tendency to reduce bonding in a sheet more than other common papermaking fillers, such as chalk, GCC and clay, and also gives the sheet an open structure which makes the sheet overly permeable or porous. As the content of PCC is increased in the furnish the demand for sizing chemicals, such as alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA) is increased to maintain the desired degree of sizing or water repellence. This is because a disproportionate fraction of the sizing chemical is adsorbed on the high surface area PCC. Poor sizing efficiency and loss of water repellence over time (size reversion) are common problems associated with the use of PCC in highly-filled, wood-free papers sized with AKD and ASA. In recent years many paper mills making wood-containing paper grades have converted to neutral papermaking to allow use of bright calcium carbonate fillers, such as GCC and PCC; major concerns with the use of calcium carbonate in these grades of paper, remain in the areas of retention, sheet strength and printing operations.

An ongoing industry trend is to decrease sheet grammage to reduce costs. Unfortunately, as the grammage is decreased, nearly all paper properties deteriorate including the limiting factors of opacity, bending stiffness and permeability. Reduction in grammage may also decrease retention of filler during papermaking and increase the frequency of sheet breaks both on the paper machine and during converting and printing. To overcome the loss in sheet opacity the papermaker can add more high opacity filler, but this can cause further deterioration in sheet strength. The industry needs cost-efficient technology for the production of lightweight grades with good filler retention and drainage and acceptable strength, formation, optical, and printing properties.

SUMMARY

Thus, in one aspect the invention relates to a filler suspension for use in papermaking, comprising filler particles; swollen cationic starch; and anionic, water-soluble polymer.

In an aspect of this invention the anionic, water-soluble polymer is lightly cross-linked.

In an aspect of this invention, the anionic, lightly cross-linked, water-soluble polymer has a tan delta rheological oscillation value at 0.005 Hz of at least 0.5 in a 1.5% aqueous solution.

In an aspect of this invention the anionic, lightly cross-linked, water-soluble polymer comprises sodium acrylate, acrylamide and methylenebisacrylamide.

In an aspect of this invention, the anionic, water-soluble polymer is TELIOFORM® M305.

In an aspect of this invention, the filler particles are selected from the group consisting of clay, talc, ground calcium carbonate, chalk, precipitated calcium carbonate, precipitated calcium sulfate, and combinations thereof.

In an aspect of this invention, the cationic starch granules are swollen with hot water at the gel point temperature of the cationic starch ±10° C., without cooking the cationic starch.

In an aspect of this invention the temperature at which the cationic starch granules are swollen is the gel point temperature +10° C.

In an aspect of this invention, the filler particles have a size of 1-10 microns.

In an aspect of this invention, the swollen cationic, starch granules have a size of 25-100 microns.

In an aspect of this invention, the filler particles are present in an amount of 60 to 99.5% by weight, the swollen, cationic starch is present in an amount of 35 to 0.499% by weight and the anionic, water-soluble polymer is present in an amount of 5.0 to 0.001% by weight based on the total solids content of the filler suspension.

An aspect of this invention is a pulp furnish comprising pulp fiber and the filler suspension herein.

In an aspect of this invention, the pulp furnish further comprises a co-additive selected from the group consisting of a sizing agent, a wet strength agent, a retention aid and combinations thereof.

An aspect of this invention is a paper comprising the above pulp furnish.

An aspect of this invention is a process for producing a filler suspension for papermaking, comprising contacting filler particles with swollen cationic starch and anionic, water-soluble polymer.

In an aspect of this invention, in the above process the anionic, water-soluble polymer is lightly cross-linked.

An aspect of this invention is a process of making paper comprising: contacting pulp fibers with the filler suspension herein to form a pulp furnish; draining the pulp furnish through a wire to form a sheet; and drying the sheet.

In an aspect of this invention, in the above process the pulp furnish further includes a co-additive selected from the group consisting of a sizing agent, a wet strength agent, a dry strength agent, a retention aid and combinations thereof.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic of a paper-making process wherein anionic, water-soluble polymer; cationic swollen starch; and filler are mixed together essentially simultaneously.

FIG. 2 is a schematic of a paper-making process wherein anionic, water-soluble polymer is premixed with swollen cationic starch before mixing with filler.

FIG. 3 is a schematic of a paper-making process wherein anionic, water soluble polymer is premixed with filler before mixing with swollen cationic starch.

FIG. 4 is a schematic of a paper-making process wherein swollen cationic starch is premixed with filler before mixing with anionic, water-soluble polymer.

FIG. 5 is a graph of the viscosity response of corn starch when heated to and beyond its gel temperature.

FIG. 6 comprises microscopic images of corn starch granules as they are heated in water at various temperatures.

DETAILED DESCRIPTION General

Use of the singular herein includes the plural and vice versa unless expressly stated to be otherwise. That is, “a” and “the” refer to one or more of whatever the word modifies. For example, “a lightly cross-linked polymer” includes one such polymer, two such polymers or, under the right circumstances, an even greater number of such polymers. By the same token, words such as, without limitation, “sizing agents” may refer to a plurality of such agents or simply to one such agent, unless, again, it is expressly stated or obvious from the context that such is not intended.

Words of approximation such as, without limitation, “about,” “substantially,” “essentially” and “approximately” mean that the feature so modified need not be exactly that which is expressly described but may vary from that written description to some extent. The extent to which the description may vary will depend on how great a change can be instituted and have one of ordinary skill in the art recognize the modified feature as still having the required characteristics and capabilities of the unmodified feature. In general, but subject to the preceding discussion, an expressly stated or implied value herein that is modified by a word of approximation may vary from the stated value by ±15%.

Compositions

In one embodiment herein there is provided a filler suspension for use in papermaking comprising filler particles, a swollen cationic starch and an anionic water-soluble polymer in a liquid vehicle, typically water.

In another embodiment, there is provided a pulp furnish comprising, in an aqueous vehicle, a filler suspension as set forth herein and pulp fibers. The furnish may also contain other papermaking agents.

In still another embodiment, there is provided a method of producing paper by adding a filler suspension of this invention to a pulp fiber stock to form a pulp furnish and then manufacturing paper from the furnish. Anionic and cationic agents can be added to the furnish containing the filler suspension to enhance retention and improve drainage. As noted previously, the furnish may also contain other papermaking agents.

The invention also provides processes for producing swollen starch/polymer compositions and their combination with filler particles to form a filler suspension.

As used herein, “swollen” starch reefers to starch in which raw starch granules have absorbed water and have expanded, preferably at present to a state in which no further water can be absorbed with rupturing the swollen granules. To accomplish this, swelling of the starch is performed under carefully controlled conditions of temperature, pH, mixing, and time. These parameters will differ from starch type to starch type and are usually determined empirically for each type of starch before being used in mill-scale paper production. The general procedure is simply to suspend raw starch in cold water and then heat the suspension until the starch is swollen. The swollen starch is then mixed with an anionic, water-soluble polymer and filler particles in any desired order to form a filler suspension. For example, in FIG. 1, swollen cationic starch 1, anionic polymer 8 a and filler particles 2 are mixed together in mixer 4 to form a filler suspension which in then transferred to mixer 5, where it is mixed with pulp fiber 3 to form a furnish. In FIG. 2, swollen cationic starch 1 and anionic, water soluble polymer 8 a are premixed and then this combination is mixed with filler particles 2 in mixer 4 after which the formed filler suspension is mixed with pulp fiber 3 in mixer 5. In FIG. 3, anionic water-soluble polymer 8 a is premixed with filler particles 2 and that combination is mixed with swollen cationic starch 1 in mixer 4 and the formed filler suspension is transferred to mixer 5 where it is mixed with pulp fiber 3 to form a furnish. In FIG. 4, swollen cationic starch 1 and fiber particles 2 are premixed in mixer 4 and the premix is transferred to mixer 5, anionic polymer 8 a being added to the premix essentially in transit between mixer 4 and mixer 5. In each of these situations, the furnish comprising the filler suspension is subsequently transferred to paper machine 6 to form paper 9. Co-additives 7 can optionally be added. During the paper drying operation, retained swollen starch granules will rupture, liberating amylopectin and amylose macromolecules, which operate to bond the solid components of the sheet.

The combination of swollen cationic starch, anionic polymer and filler particles can be used for papermaking under acid, neutral or alkaline conditions. The compositions are used primarily to assure that the filler and starch are well-retained in paper sheets during the paper-making process while having a minimal negative effect on sheet strength. Using swollen cationic starch/anionic water-soluble polymer/filler particle compositions tends to result in greater retention and strength than using swollen cationic starch or anionic polymer alone with the filler particles.

When the filler is mixed with a swollen cationic starch/anionic water-soluble polymer and added to a pulp slurry, the filler particles agglomerate and adsorb onto the surface of the slurry fines and fibers causing rapid flocculation in the furnish. This can result in good retention of the filler and fines and improves web drainage even without the addition of a retention aid. Under high levels of shear, turbulence and vacuum, however, filler retention can be reduced due to deflocculation and detachment of the filler from the fiber surfaces. Adding an anionic micro-particle, such as colloidal silicic acid, to the papermaking furnish containing the filler suspension, at or prior to the headbox, and preferably at present to the pressure screen of the paper machine, can further enhance retention and drainage.

Fillers

The filler particles can be any known to those of skill in the art and the filler suspension can comprise a single filler or more than one filler. The fillers particles are typically inorganic materials having an average particle size ranging from 0.5 to 30 μm, more usually 1 to 10 μm, such as, without limitation, clay, ground calcium carbonate (GCC), chalk, precipitated calcium carbonate (PCC), talc, and precipitated calcium sulphate (PCS) and their blends. At present PCC is a preferred filler. The pulp slurry to which the filler suspension is added can be composed of mechanical pulp, chemical pulp, recycled pulp and mixtures thereof.

Cationic Starch

Starches suitable for use in this invention include, without limitation, those originating from corn, waxy corn, potato, wheat, tapioca, sorghum, waxy sorghum and rice. The starches are generally rendered cationic by inclusion of quarternary ammonium cations in the starch. Cationic starches are commercially available and their preparations are well-known to those skilled in the paper-making art and therefore need not be further described herein. The average particular size of most raw starch granules is about 5 to 45 μm.

Starch granules are insoluble in cold water. To disperse or “cook” starch, the starch is heated in aqueous suspension. As heating proceeds, the starch granules first go through a stage of slight, reversible swelling until a critical temperature, referred to as the “pasting,” “gelatinization” or simply “gel” temperature, massive swelling occurs, which causes a large increase in viscosity. If held for a sufficient period above the gel temperature, the viscosity reverts to lower levels due to the rupture of the swollen granules. Each variety of starch has its own gel temperature. The gel temperature for many starches is available in extant literature or it can be readily empirically determined by heating a given starch suspension while monitoring viscosity. Swollen starch granules are distinct from cooked starch. Cooked starch results when swollen starch granules rupture at temperatures above the gel temperature and thereby release amylose and amylopectin, which dissolve in the aqueous medium.

For the purposes of this invention, swelling of starch granules is carefully controlled so as to form a swollen starch in which a minimal amount of swollen granule rupture has taken place. Depending on the starch source, the ultimate particle size of the swollen starch granules ranges from about 25 μm to about 100 μm. A representative, but non-limiting, example of a swollen starch preparation is presented in the Examples.

Anionic, Water-Soluble Polymer

The anionic, water-soluble polymer may be linear or lightly cross-linked. By lightly cross-linked is meant that the cross-linked polymer remains water soluble. That is, the lightly cross-linked polymer appears more like a “branched” polymer than a fully cross-linked polymer in which the polymer chains are inextricably intertwined and is thereby rendered insoluble in water. As used herein, lightly cross-linked and branched are used interchangeably to refer to a polymer that is cross-linked but is still water-soluble. Examples of anionic, branched, water-soluble polymers are described in U.S. Pat. Nos. 5,958,188, 6,391,156 B1, 6,395,134 B1, 6,406,593 B1 and 6,454,902 B1, which are hereby incorporated by reference in their entireties as if fully set forth herein.

In certain embodiments, the anionic, branched, water soluble polymer:

(a) is prepared by adding a cross-linking or branching agent to the monomer charge; (b) has an intrinsic viscosity above about 3 dl/g; and (c) has a tan delta rheological oscillation value at 0.005 Hz of at least 0.5, or has a deionised SLV viscosity number which is at least three times the salted SLV viscosity number of the polymer made under the same conditions from the same monomer charge but in the absence of branching agent.

The polymer can be made by reacting a monomer or monomer blend under polymerization conditions, such as reverse phase emulsion polymerization, in conventional manner with a cross-linking or branching agent included in the monomer charge. The amount of branching agent and the polymerization conditions are selected in such a manner that the polymerization results in a water soluble polymer and not a water insoluble, cross-linked polymer.

For the purpose of this invention, it is not necessary to specify a numeric range, i.e. percent, cross-linking that will result in a usable water-soluble, branched polymer. Rather, the properties of the resultant polymer are monitored until certain ranges in those properties are achieved. One indication that a branched polymer is behaving as a solution polymer rather than a microparticulate polymer is the tan delta value. While not being held to any particular theory, it is believed that, at low frequencies, such as 0.005 Hz, the rate of deformation of a polymer sample is sufficiently slow so as to enable linear or branched entangles chains to disentangle, resulting in high tan delta values. Network or heavily cross-linked polymers, on the other hand, are permanently entangled and exhibit low tan delta values across a wide range of frequencies. Thus, by determining the tan delta at low frequency, i.e, 0.005, a measure of the degree of branching can be ascertained. For the purpose of this invention, a tan delta value above 0.5, preferably above 0.7 and, even more preferably, above 0.9, 1.3 or even higher. Tan delta values below 0.5 generally indicate a polymer that is too heavily cross-linked to act as a true solution polymer and is preferably avoided.

The tan delta value at 0.005 Hz can be obtained using a Controlled Stress Rheometer in the Oscillation mode on a 1.5% by weight solution of polymer in deionised water. The value of tan delta is the ratio of the loss (viscous) modulus G″ to the storage (elastic) modulus G′.

The branched polymers herein preferably have a tan delta value at 0.005 Hz that is reasonably close to the value of the corresponding unbranched polymers; that is, polymers made under the same conditions but in the absence of branching agent, which results in a higher intrinsic viscosity. For instance, the branched polymers preferably have a tan delta which is at least 50% and often at least 80%, for instance up to 120% or more of tan delta for the corresponding unbranched polymers.

Another indication that the polymer is in solution rather than being microparticulate is that the deionised SLV viscosity number for the branched polymer is at least three times the salted SLV viscosity number of a polymer made by reacting the same monomer charge in the absence of branching agent under the same polymerisation conditions. This referred to as the “corresponding unbranched polymer.”

The “same monomer charge” and the “same polymerisation conditions” indicates that the charge and the conditions are as constant as is reasonably achievable in commercial production, except for deliberate variations in the amount of branching agent and, if appropriate, chain transfer agent.

The branching agent may be any cross-linking entity capable of reacting with a pendant group on the main polymer chain such as, without limitation, the carboxylic acid group of acrylic acid but preferably the branching agent is a polyethylenically unsaturated monomer. The polyethylenic branching agent can be a difunctional material such as methylenebisacrylamide or it can be a trifunctional, tetrafunctional or higher functional branching agent, for instance tetraallylammonium chloride. Preferably the branching agent itself is water soluble.

The amount of polyethylenic branching agent is generally below 10 ppm molar, preferably below 5 ppm molar. Useful results may be obtained with about 0.5 to 3.8 ppm molar but in some instances amounts from 4.1 up to 7 or even 10 ppm molar may be used. In fact, sometimes amounts up to 20 ppm molar or even up to 30 or 40 ppm molar (generally in the presence of chain transfer agent) may be used but lower amounts are usually needed in order to comply with the tan delta limits. The designation “ppm molar” refers to moles branching agent per million moles monomer (i.e., ppm molar).

The branched polymer may be made under polymerisation conditions wherein it is intended that there be no deliberate chain transfer agent present during the reaction. The amounts of branching agent set forth above, in particular 1 to 10 ppm molar and preferably 0.5 to 3.8 ppm molar, are especially suitable when no chain transfer agent is added. It can, however, be desirable to add some chain transfer agent, in which event it is possible to increase the amount of branching agent up to 20 or 30 ppm molar or even 40 ppm molar while still maintaining the characteristic properties and performance of the polymer. The amount of chain transfer agent selected will depend upon the particular material which is being used and upon the amount of branching agent, the monomer charge, and the polymerisation conditions.

Although quite large amounts of branching agent can be used, preferably the amount is low since it has been observed that best results are obtained with low amounts of chain transfer agent. A preferred chain transfer agent is sodium hypophosphite. Although large amounts can be used, best results generally require amounts below 50 ppm and preferably below 20 ppm (by weight based on the weight of monomer). Best results are generally obtained with not more than 10 ppm. If the amount is too low, however, such as below about 2 ppm, there may be inadequate benefit from using a chain transfer agent.

Any chain transfer agent suitable for use in the aqueous polymerisation of water soluble acrylic monomers, such as, without limitation, isopropanol or mercapto compounds, can be used as an alternative to hypophosphite. If a material other than hypophosphite is being used, it should be used in an amount that results in substantially the same chain transfer effect as the amount of hypophosphite it replaces.

Although it is preferred to use low amounts of chain transfer agent, it is possible to use larger amounts, for instance 100 ppm or more, generally with less effective results, provided that the combination of materials and polymerization conditions is such that the polymer has the desired physical properties.

One of the desirable physical properties is the intrinsic viscosity of the polymer. This is measured using a suspended level viscometer in 1 M NaCl buffered to pH 7.5 at 25° C. It is usually at least 3 or 4 dl/g, and preferably at least 6 dl/g. It can be as high as, for instance, 18 dl/g but is usually below 12 dl/g and often below 10 dl/g.

A suitable branched polymer can also be characterised by comparison to the corresponding unbranched polymer. The unbranched polymer will generally have an intrinsic viscosity of at least 6 dl/g and preferably at least 8 dl/g. Often it is 16 to 30 dl/g. The amount of branching agent is usually such that the intrinsic viscosity is reduced by at least 10% and usually at least 25% or 40%, up to 70%, or sometimes up to 90%, of the value for the unbranched polymer.

Instead of or in addition to intrinsic viscosity, the polymer can also be characterised by its saline Brookfield viscosity. The saline Brookfield viscosity is measured by preparing a 0.1% by weight solution of polymer in a 1 M NaCl aqueous solution at 25° C. A Brookfield viscometer fitted with a UL adaptor at 60 rpm can be used. Thus, powdered polymer is added to the 1 M NaCl aqueous solution or a reverse phase emulsion polymer is added to that solution. The saline solution viscosity is generally above 2.0 mPa·s and is usually at least 2.2 and preferably at least 2.5 mPa·s. Generally, it is not more than 5 mPa·s and values of 3 to 4 are usually preferred.

Instead of or in addition to tan delta values as an indicator of the absence of insoluble cross-linked microparticles, it is also possible to use the ratio between deionized and salted SLV viscosity numbers.

The SLV viscosity numbers are determined by use of a glass suspended level viscometer at 25° C., the viscometer being chosen according to the viscosity of the solution. The viscosity number is η−η₀/η₀ where η and η₀ are the viscosity results for aqueous polymer solutions and solvent blank, respectively. This can also be referred to as specific viscosity. The deionized SLV viscosity number is the number obtained for a 0.05% solution of the polymer in deionised water. The salted SLV viscosity number is the number obtained for a 0.05% solution of polymer in 1 M aqueous sodium chloride.

The deionized SLV viscosity number is preferably at least 3 and generally at least 4. Best results are obtained when it is above 5 such as 7 or 8. Preferably it is higher than the deionized SLV viscosity number for the unbranched polymer. If the deionised SLV viscosity number is not higher than the deionized SLV viscosity number of the unbranched polymer, preferably it is at least 50% and usually at least 75% of the deionized SLV viscosity number of the unbranched polymer. The salted SLV viscosity number is usually below 1. The deionized SLV viscosity number is often at least five times, and preferably at least eight times, the salted SLV viscosity number.

The polymers may be obtained from commercial sources but they can also be made by any of the conventional suitable polymerization processes which are known for making water soluble acrylic and other addition polymers such as bead or gel polymerizations. The preferred type of polymerization process is reverse phase emulsion polymerization so as to form a reverse phase emulsion of water soluble polymer particles in non-aqueous liquid. This product typically has an initial particle size at least 95% by weight below 10 μm and preferably at least 90% by weight below 2 μm, for instance down to 0.1 or 0.5 μm. It can therefore be a conventional reverse phase emulsion or microemulsion and can be made by any of the known techniques for making such materials. If desired the number average size can be typical of a microemulsion, for instance down to 0.05 or 0.1 μm.

The emulsion can be supplied in the form in which it is made (as an emulsion of aqueous polymer droplets in oil or other water immiscible liquid) or, if desired, it can be substantially dehydrated to form a stable dispersion of substantially anhydrous polymer droplets dispersed in oil. Conventional surfactant and optional polymeric amphipathic stabiliser may be included in known manner to stabilize the emulsion.

The reverse phase or other polymerization process is conducted on a charge of the desired monomer or monomer blend usually in aqueous solution.

It is generally preferred that the anionic, branched polymer be a copolymer of 5 to 97% by weight acrylamide or other water soluble, non-ionic, ethylenically unsaturated monomer and 95 to 3% by weight ethylenically unsaturated carboxylic, sulfonic or other anionic monomer. Any conventional water-soluble carboxylic and sulfonic monomers may be used such as, without limitation, acrylic acid, methacrylic acid, crotonic acid, vinyl sulfonate and AMPS. A presently preferred anionic monomer is acrylic acid, often as sodium acrylate or other water soluble salt. Preferred copolymers contain from 20 to 80%, often 40 to 75% by weight acrylic acid with the balance being acrylamide.

In particular embodiments, the anionic, water-soluble polymer is TELIOFORM® M305 (BASF, commercially available).

Swollen Cationic Starch/Anionic Polymer/Filler Particle Suspension

In general, the suspension will comprise 60 to 99.5% by weight filler, 35 to 0.499% by weight swollen cationicstarch and 5.0 to 0.001% by weight anionic polymer to a total of 100% based on the total solids content of filler particles, cationic starch and anionic polymer. It is understood that the suspension will contain complexes of anionic polymer bound to swollen cationic starch, but may also contain free swollen cationic starch and free anionic polymer particles.

The swollen cationic starch and anionic polymer are suitably employed in an amount of 0.5 to 10% by weight as dry solids, based on the weight of filler particles.

Papermaking Agents

The compositions, suspensions and furnishes herein may additionally include conventional papermaking agents such as, without limitation, sizing agents such as alkylketene dimer, alkenyl succinic anhydride and rosin; wet strength agents, and cationic or anionic polymeric retention aids. The composition may include a retention aid which may be a single chemical, such as an anionic micro-particle (colloidal silicic acid, bentonite), anionic polyacrylamide, a cationic polymer (cationic polyacrylamide, cationic starch), dual chemical systems (cationic polymer/anionic micro-particle, cationic polymer/anionic polymer) or three component systems (cationic polymer/anionic microparticle/anionic polymer, cationic polymer/anionic micro-polymer/anionic polymer). The choice of retention aid chemicals and their addition points in the paper-forming process will depend on the nature of the ionic charge of the treated filler slurry and the papermaking furnish.

In general, the filler suspension herein is used in an amount of 5% to 60%, as dry solids, based on the dry weight of pulp in furnish.

Paper sheets made with filler can exhibit greater internal bond strength, as measured by the Scott bond technique, than a control sheet made with no filler. At equal filler content, the wet and dry strength properties of sheets made using the filler suspension herein can be greater than those sheets made with the filler alone.

The use of the filler suspension of this invention permits the production of filled papers, such as coated and uncoated fine papers, super-calendered papers, and newsprint, with minimal strength loss and good optical properties. Using the filler suspension of this invention can thus allow papermakers to produce filled papers with a higher filler content in the paper sheet. In general, the potential benefits from the use of the treated filler suspensions of the present invention include improved sizing, wet strength, dry strength, opacity and print quality, and reduced use of expensive reinforcement chemical pulp fiber.

Under certain conditions the combination of swollen cationic starch and anionic polymer may be used to strengthen other grades that contain no filler such as sack papers and paperboard products.

EXAMPLES General

The suspensions and methods of this invention can be described and understood by the following illustrative examples. In the examples, the results were obtained using laboratory scale techniques. These examples, however, are not intended nor are they to be construed to be limiting in any manner whatsoever.

The starch, in a slurry at 2-20% solids, at room temperature may be swollen at temperatures approximately the starch gel point in a batch cooker, a jet cooker or by mixing with hot water. The preferred method is to swell the granules by mixing the starch slurry prepared in cold water with hot water. The temperature of hot water used depends on the consistency of the initial starch slurry in cold water, the final target temperature of the swollen starch, the temperature of the cold water, pH, and residence time. The temperature and reaction time for preparing the swollen starch/polymer composition depend on the type of starch used, the pH of the starch slurry and heating time. The following are examples of processes for the preparation of swollen starch for the purposes of this invention.

Example 1

A raw starch dispersion mixed with polymer in cold water is swelled and the swollen starch/polymer composition is added to an agitated filler suspension. In this method, the starch powder is first dispersed in cold water then polymer is incorporated into the dispersion under shear. The starch/anionic polymer mixture is mixed with hot water or is heated to a temperature that is approximately the starch gel point. The swollen starch/anioic polymer composition is then rapidly mixed with the filler suspension at a temperature below the starch gelation temperature.

Example 2

A cationic starch dispersion is first swelled, then added to an agitated filler suspension followed by the introduction of anionic polymer. In this method, the starch powder is dispersed in cold water then mixed with hot water or heated to a temperature that is approximately the starch gel point. The swollen starch is then rapidly mixed with the filler suspension at a temperature below the starch gel point followed by addition of anionic polymer.

The combination of swollen cationic starch, anionic, water-soluble polymer and filler are performed under good mixing conditions. Other anionic agents or cationic agents can be added during the preparation of swollen cationic starch/anionic water-soluble polymer combination to form a complex prior to the addition of filler or they can be added to the sheared treated filler suspension to develop bridged filler particles. These treatment strategies can produce homogeneous filler suspensions, which are stable during storage for a long period.

The treated filler suspensions can be introduced directly into the pulp slurry or, if desired, can be diluted and added to the paper machine pulp stock prior to the sheet forming process, e.g., at the blend chest, machine chest, or inlet of the fan pump. The introduction of the treated filler to the pulp suspension induces flocculation of the pulp slurry. The degree of flocculation is, however, influenced by the level of shear and residence time. In general, the treated-filler suspensions tend to retain their flocculation characteristics over time when added to papermaking pulp slurries. To enhance filler retention an anionic micro-particle, such as, without limitation, silica, an anionic polymer such as, without limitation, CMC, or a conventional polymeric retention aid such as, without limitation, polyarylamide, can be added to the furnish, preferably at a point prior to or at the headbox or pressure screen. Upon addition of silica or CMC to the pulp stock containing the treated filler, retention and drainage substantially improved.

Example 3

A 0.3% concentration stock was prepared by mixing internal cationic starch with pulp furnish followed by pretreated PCC and finally retention aid. 80 g/m² wood-free handsheets were made using dynamic sheet former (DSF) followed by dynamic sheet pressing and drying at 120° C. Prior to paper testing the paper sheets were calendered under the same conditions and then conditioned at 50% RH and 22° C.

The raw materials used in the sheet making were following:

Fiber: 100% eucalyptus used as pulp, refined to SR 30 (at 20° C.) using a Valley Beater lab refiner. Filler: precipitated calcium carbonate (Albacar LO PCC) from Specialty Minerals Inc., average particle size 2.3 μm. The PCC content in the sheets varied between 19.3 and 25.9% by weight. Swollen cationic starch used for PCC pretreatment: Cationic potato starch. Swollen cationic starch was prepared by mixing dry cationic starch powder with water to make 3% solids slurry, which was then heated to 63° C. under mixing. Swollen cationic starch was used for PCC pretreatment by mixing 5 kg/metric ton (tonne) paper of swollen cationic starch with PCC at 20% solids. Some filler samples were only pretreated with swollen cationic starch and some were pretreated with swollen starch and anionic co-additive. Co-additive used for PCC pretreatment with swollen cationic starch: anionic micropolymer (TELIOFORM M305®, trade-mark of BASF). The treatment of PCC was done by mixing swollen cationic starch and anionic micropolymer with PCC. Different addition orders were used and while the swollen cationic starch dose was fixed at 5 kg/tonne paper, the anionic micropolymer doses were 0.05% as received material/dry PCC weight and 0.1% as received material/dry PCC weight. Internal starch: Cationic potato starch. Dry starch powder was mixed with water to get a 1% solids slurry, which was then cooked at 97° C. under mixing. Cooked cationic starch was used at 8 kg/paper tonne by mixing it with pulp furnish. Retention aid: 0.2 kg/paper tonne of cationic polyacrylamide (CPAM) was used for retention.

Example 4

Table 1 presents the properties of the sheets made with only swollen starch treated PCC and swollen starch followed by anionic micropolymer treated PCC. Anionic micropolymer doses were 0.05-0.1% as received material/dry PCC weight. The sheets having anionic micropolymer in PCC pretreatment show better strength properties—tensile, internal bond, bending stiffness—compared to swollen starch alone treated PCC. The best strength performance was achieved with highest 0.1%/PCC anionic micropolymer dose. This would allow 6% unit filler increase without loss in strength properties.

TABLE 1 Bending PCC Tensile Index, Internal Bond, stiffness, content, % Nm/g J/m² mNm PCC treated 19.3 28.0 235 0.18 with 5 kg/tn SST PCC treated 19.7 28.5 257 0.20 with 5 kg/tn 23.6 26.5 239 0.18 SST and 0.05% AMP PCC treated 20.4 33.5 278 0.24 with 5 kg/tn 25.2 29.8 267 0.20 SST and 0.1% AMP PCC: precipitated calcium carbonate, SST: cationic swollen starch, AMP: anionic micropolymer. Tensile and stiffness values are geometrical averages from machine and cross directions.

Example 5

Table 2 presents the properties of the sheets made with only swollen starch treated PCC, and swollen starch and anionic micropolymer treated PCC using different addition orders: PCC treated with swollen starch followed by anionic micropolymer and anionic micropolymer treated PCC followed by swollen starch. The presence of anionic micropolymer improves the strength properties of the sheets made with swollen starch treated PCC independently of the addition order.

TABLE 2 PCC Tensile Index, Internal Bond, content, % Nm/g J/m² PCC treated with 5 kg/tn 19.3 28.0 235 SST PCC treated with 1) 5 kg/tn SST and 20.4 33.5 278 2) 0.1% AMP 25.2 29.8 267 PCC: precipitated calcium carbonate, SST: cationic swollen starch, AMP: anionic micropolymer Tensile and stiffness values are geometrical averages from machine and cross directions.

Example 6

These microscope images in FIG. 6 illustrate how starch granules swell and how the viscosity increases until it starts to decrease due to rupture of swollen starch granules. The images represent samples of corn starch at 25° C., 56° C., 60° C., 66° C. and 95° C.

For the purpose of this invention, swollen starch is characterized where most of the granules have started to swell as in the 56° C. image up to point where large swollen granules are still visible as seen in the 66° C. image. Thus, the viscosity curve, together with microscopic images, can be used to determine when a starch is swollen for use in preparing the filler suspension of this invention. The maximum viscosity area in FIG. 5 is where most of the starch granules are swollen but not ruptured. The temperature range within which useful swollen starch granules can be obtained is in the peak region of FIG. 5+/−10° C., where most of the starch granules are swollen but not yet fully ruptured. Preferable the temperature at which the raw starch granule suspension is heated is the peak +10° C. where all the starch granules are swollen and all unswollen granules eliminated.

All patents and patent applications cited in this specification are Incorporated by reference as if each individual patent and patent application were specifically and individually fully set forth herein. While the claimed subject matter has been described in terms of various embodiments, the skilled artisan will appreciate that various modifications, substitutions, omissions, and changes may be made without departing from the spirit thereof. Accordingly, it is intended that the scope of the subject matter limited solely by the scope of the following claims, including equivalents thereof. 

What is claimed is:
 1. A filler suspension for use in papermaking, comprising: filler particles; swollen cationic starch; and anionic, water-soluble polymer.
 2. The filler suspensions of claim 1, wherein the anionic, water-soluble polymer is lightly cross-linked.
 3. The filler suspension of claim 2, wherein the anionic, lightly cross-linked, water-soluble polymer has a tan delta rheological oscillation value at 0.005 Hz of at least 0.5 in a 1.5% aqueous solution.
 4. The filler suspension of claim 2, wherein the anionic, lightly cross-linked, water-soluble polymer comprises sodium acrylate, acrylamide and methylenebisacrylamide.
 5. The filler suspension of claim 1, wherein the anionic, water-soluble polymer is TELIOFORM® M305.
 6. The filler suspension of claim 2, wherein the anionic, water-soluble polymer is TELIOFORM® M305.
 7. The filler suspension of claim 1, wherein the filler particles are selected from the group consisting of clay, talc, ground calcium carbonate, chalk, precipitated calcium carbonate, precipitated calcium sulfate, and combinations thereof.
 8. The filler suspension of claim 1, wherein the cationic starch granules are swollen with hot water at the gel point temperature of the cationic starch ±10° C., without cooking the cationic starch.
 9. The filler suspension of claim 8, wherein the temperature at which the cationic starch granules are swollen is the gel point temperature +10° C.
 10. The filler suspension of claim 1, wherein the filler particles have a size of 1-10 microns.
 11. The filler suspension of claim 1, wherein the swollen cationic, starch granules have a size of 25-100 microns.
 12. The filler suspension of claim 1, wherein the filler particles are present in an amount of 60 to 99.5% by weight, the swollen, cationic starch is present in an amount of 35 to 0.499% by weight and the anionic, water-soluble polymer is present in an amount of 5.0 to 0.001% by weight based on the total solids content of the filler suspension.
 13. A pulp furnish, comprising pulp fiber and the filler suspension of claim
 1. 14. The pulp furnish of claim 13, further comprising a co-additive selected from the group consisting of a sizing agent, a wet strength agent, a retention aid and combinations thereof.
 15. A paper comprising the pulp furnish of claim
 13. 16. A process for producing a filler suspension for papermaking, comprising contacting filler particles with swollen cationic starch and anionic, water-soluble polymer.
 17. The process of claim 16, wherein the anionic, water-soluble polymer is lightly cross-linked.
 18. A process of making paper comprising: contacting pulp fibers with the filler suspension of claim 1 to form a pulp furnish; draining the pulp furnish through a wire to form a sheet; and drying the sheet.
 19. The process of claim 18, wherein the pulp furnish further includes a co-additive selected from the group consisting of a sizing agent, a wet strength agent, a dry strength agent, a retention aid and combinations thereof. 